All farm land that has been cultivated (plowed) or had any traffic on it will form a compacted layer of soil beneath the surface. This compacted layer is called a "plow pan" or "hard pan". The "hard pan" is usually two to four inches thick and is located normally six to fourteen inches beneath the surface. On cultivated land, the "hard pan" depth is determined by the depth of plowing. Two factors contribute to the development of a hard pan. One is merely the weight of the implements used to till the soil. The other is called a "traffic pan". The traffic pans are created under the wheels of the farm tractor because of its weight. As the traffic pans are plowed, they move further down until they reach a depth where a plow will not disturb them. They there join the compaction caused by the tillage implements to form the hard pan. When soil is loose, it will be compacted to within 88 percent of maximum density by one pass across it with an average size farm tractor. Therefor all successive passes compact it only 12 percent higher (tighter). This hard pan severly restricts plant root growth. If it is compacted tightly enough, roots cannot penetrate it and therefore plant growth and yields are greately restricted. The density of compaction is determined to some extent by the type of soil. With no hard pan or other restriction, a plant's roots may grow to a depth of 6 feet and a width of 4 feet within 30 days after its seed germinates. However, when a hard pan is such that roots cannot penetrate it, the plant must obtain all its nutrients and moisture from the layer of earth above the hard pan. Therefore, it must receive rain at frequent intervals in order to survive. But it can sustain itself for a longer period of dry weather if it is feeding off of 6 feet of earth instead of merely of that above the hard pan.
Although there have heretofore been sub-soil plowing implements, the plow itself tends to pack the earth on either side of the blade thereof, as well as the subsequent traffic of farm vehicles as tractors and implements drawn thereby such as other separate vehicles carrying separately rollers to pack dirt into the deeply-plowed groove as well as the wheels of the vehicle itself (themselves) serving to further and again pack the soil such that as before the roots only with difficulty are able to penetrate the packed soil, if at all, the problem heretofore has not been solved by such sub-soil plowing and filling-up procedures.